1. Field of the Invention
The present invention relates to a power generating device.
2. Description of the Related Art
A power generating device for driving a power generator by rotating a turbine with steam is widely used. Secondarily utilizing low pressure steam discharged from the turbine is disclosed in Japanese Patent Laid-Open Nos. 2006-2576 and 2004-100657. In the power generating device in which the turbine is used, it is difficult to control pressure of the low pressure steam discharged from the turbine.
A possibility of utilizing a positive displacement steam expander such as a screw expander is examined. The positive displacement steam expander has a flat characteristic that torque is determined on the basis of not a rotation speed but a difference between a supply pressure and a discharge pressure, and a flow rate of steam is proportional to the rotation speed.
Output of the positive displacement steam expander is represented by the product of the torque and the rotation speed. In a design of the power generating device, the power generator is selected so as to maximize a differential pressure between the supply pressure and the discharge pressure of the steam, and obtain rated output at the time of operating with a rated frequency.
In accordance with an operating state of equipments demanding low pressure steam in secondary side of the power generating device, or an operating state of devices arranged in parallel to the power generating device and utilizing the steam, there is sometimes a case where a pressure of the secondary side steam is increased and the differential pressure in the positive displacement steam expander is decreased so that the output of the power generator is lowered.
With regard to the frequency of the power generating device, for example Japanese Patent Laid-Open No. 2005-176496 describes a technique of controlling the rotation speed of the turbine by setting the frequency of the power generator.